Japanese Patent Laying-Open No. 2004-203219 has disclosed a hybrid driving apparatus in which an electric motor allowing control of a rotational speed is connected to an output member receiving a torque supplied from a main power source via a transmission mechanism performing shifting by changing an engagement device between engaged and released states.
This hybrid driving apparatus controls the electric motor to attain a specific target rotational speed during power-off shifting so that a shifting delay and/or a shifting shock may be prevented at the time of power-off shifting.
Japanese Patent Laying-Open No. 06-319210 has disclosed another hybrid driving apparatus that can prevent a shock at the time of shifting by executing torque down of an electric motor when a torque is being transmitted from a rotation output shaft to the electric motor during shifting.
Further, it can be expected that the shift shock can be prevented in a wider range of the drive state when the technology disclosed in Japanese Patent Laying-Open No. 06-319210 for preventing the shift shock due to the torque down of the electric motor is applied to the hybrid driving apparatus of Japanese Patent Laying-Open No. 2004-203219.
In some cases, charging is limited in a power storage device connected electrically to a power generator. More specifically, when the power storage device is in a state of charge of a rated value or more, the charging is limited for protecting the power storage device from overcharge.
As described above, when the torque down of the electric motor is executed in the hybrid driving apparatus that disclosed in Japanese Patent Laying-Open No. 2004-203219 and further employing the technology of Japanese Patent Laying-Open No. 06-319210, power consumption of the electric motor decreases so that a surplus power in the power generated by the power generator increases. When the charging of the power storage device is limited in this case, it is necessary to avoid the increase in quantity of the power generated by the power generator for protecting the power storage device. Accordingly, it may be envisaged to employ a manner that executes the torque down of the power generator along with the torque down of the electric motor for keeping a power balance in the hybrid driving apparatus.
However, output control of a main power source (e.g., an engine) is performed relatively slowly as compared with torque down control of the electric motor and power generator so that a power balance is lost in the hybrid driving apparatus. This results in rising of the rotational speed of the main power source.
Usually, an allowed upper limit rotational speed is determined in the engine, and the rotational speed must be kept at or below the allowed upper limit rotational speed. Therefore, it is considered that the foregoing torque down of the electric motor and power generator may raise the rotational speed of the main power source, i.e., the engine to exceed the allowed upper limit rotational speed.
As described above, it is difficult to achieve simultaneously two mutually contradictory objects, to avoid the shift shock due to execution of the torque down of the electric motor and power generator and to keep the main power source at or below the allowed upper limit rotational speed. In this case, it can be considered that the latter is achieved on a priority basis for preventing damages of the main power source, resulting in a problem that the shift shock cannot be avoided.